This invention relates to cross-blade flexures, and, more particularly, to the fabrication of a cross-blade flexure block whose stiffness properties and deflection symmetry are highly regular and predictable.
A cross-blade flexure block is a known device which acts as a torsional spring. It provides a well-defined torsional response about its housing axis and is desirably resistant to defection about other axes. The cross-blade flexure block provides this response in a compact size, so that it is particularly useful in cases where the entire structure is constrained to lie within a small volume.
An example of an application is a mirror mount of a beam-steering mirror. Two of the cross-blade flexure blocks are mounted with their housing axes in the plane of the mirror surface and orthogonal to each other. Drive motors deflect the mirror about the axes, and the cross-blade flexure blocks provide a measured restraint to the deflection and a restoring force to the null position.
For this and other applications, the angular deflection as a function of the applied torque must be very precisely known and predictable. Because of the complexity of the geometry of the cross-blade flexure block, the conventional practice is to design the configuration of the cross-blade flexure block using finite element design techniques. The angular deflection as a function of force is simulated for varying geometries and materials of construction. When the desired characteristics have been achieved, the geometry is transferred to the final drawings.
To build each cross-blade flexure element from which the cross-blade flexure block is assembled, the two halves of the housing of the cross-blade flexure element and the flexing blade are machined. The two halves are then joined by brazing one end of the blade to each of the housing halves in the proper geometry. A number of the cross-blade flexure elements are assembled together to form the cross-blade flexure block.
Upon testing, it is found that many of the individual cross-blade flexure elements do not conform to the predictions of the analytical procedure of angular deflection as a function of applied torque. In small-scale, speciality applications, the user is often forced to use the nonconforming cross-blade flexure block. In mass production circumstances, the nonconforming cross-blade flexure elements are discarded, and only the conforming cross-blade flexure elements are assembled into the final cross-blade flexure block. The cost of each cross-blade flexure block is therefore quite high, because of the low production yields. No approach has been suggested to improve the ability to produce cross-blade flexure blocks that more closely conform to the predicted properties, or to improve yields to achieve lower total production costs.
There is a need for an improved approach to the production of cross-blade flexure blocks, which achieves acceptable, predictable performance with lower total production costs. The present invention fulfills this need, and further provides related advantages.
The present invention provides an improved approach to the production of cross-blade flexure elements, and thence cross-blade flexure blocks. The stiffness properties and deflection symmetry of the cross-blade flexure blocks are highly regular and predictable by available design procedures. The cross-blade flexure blocks have acceptable performance at a cost that is on the order of one-sixth that of cross-blade flexure blocks made by the conventional approach. The production techniques used in the present approach are widely available, as distinct from the narrower availability of the production techniques used in other approaches.
In accordance with the invention, a method of fabricating a cross-blade flexure block comprises the steps of furnishing a first plate of a flexure material, and machining a first cross-blade flexure element from the first plate as a first single piece of material. The method further includes furnishing a second plate of the flexure material, and machining a second cross-blade flexure element from the second plate as a second single piece of material. (The first plate and the second plate may be the same plate or different plates, but they are of the same nominal material.) The first cross-blade flexure element and the second cross-blade flexure element are assembled together to form a flexure block. In the assembly, locating pins may be positioned between the first cross-blade flexure element and the second cross-blade flexure element.
In a preferred case, each of the cross-blade flexure elements comprises an upper blade housing, and a lower blade housing positioned with respect to the upper blade housing such that there is a planar slot between the upper blade housing and the lower blade housing and lying in a slot plane. The upper blade housing and the lower blade housing are shaped to define an external form factor symmetric about a housing axis (which is the torque axis in service) and a bore therethrough symmetric about the housing axis. A blade extends transversely through the bore between the upper housing and the lower housing and intercepts the housing axis. The blade has a blade angle of from more than 0 to less than 90 degrees relative to the slot plane. Usually, the blade angle is about 45 degrees. Preferably, the first cross-blade flexure element and the second cross-blade flexure element have the same shape, except that the blade angle of the first cross-blade flexure element is +A, and the blade angle of the second cross-blade flexure element is xe2x88x92A, relative to the slot plane.
Because of the complexity of this structure and its small size for most applications, it has been conventional practice to machine the two blade housings and the blade as separate components and then join them by brazing. The inventor has discovered that the variability observed in the final products results from variations in this production process, particularly the variations resulting from the brazing operation. Although it is widely and successfully used in many applications, brazing is simply not a sufficiently well-controlled process to produce the very exact properties needed in the final structure of the cross-blade flexure elements and assembled blocks.
The inventor has therefore determined to design the cross-blade flexure elements using finite-element analysis. The cross-blade flexure elements are fabricated as a single piece using an appropriate machining technique such as electrical discharge machining (EDM) or high-pressure water-jet machining, guided by numerical-control parameters established in the finite-element analysis. Eliminating the three-piece structure of the conventional cross-blade flexure element, and particularly the brazing operation used to join the three pieces together, has resulted in much less variability of properties and better predictability of properties. The manufacturing yield of the cross-blade flexure elements is therefore much greater than achieved by conventional manufacturing techniques, and the total and per-finished-part manufacturing costs are reduced substantially. Initial studies indicate that the costs in the present approach are reduced to one-sixth or less of the costs of conventionally produced cross-blade flexure blocks.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.